When a fluoropolymer is produced by emulsion polymerization, carboxyl group formation occurs at one or both main chain ends thereof due to the use of such a polymerization initiator as a persulfate salt. This carboxyl group is thermally unstable and is decomposed on the occasion of melt molding, generating a volatile matter and thus deteriorating the moldability of the fluoropolymer. Therefore, it is necessary to stabilize this unstable terminal group prior to subjecting the fluoropolymer to molding.
An old known method of stabilizing carboxyl group-containing fluoropolymers comprises adding water to a powder as obtained after polymerization and heating the mixture at an elevated temperature (cf. e.g. Patent Document 1: Japanese Kokoku Publication S37-3127). In this document, it is alleged that the addition of an alkali metal promotes the conversion of the carboxyl group to stable —CF2H and, even in the absence of any alkali metal, the carboxyl group is converted to —CF2H. However, this method has a problem, namely it allows a large number of carboxyl groups not yet converted to —CF2H to remain.
Also known as methods of stabilizing the carboxyl group by conversion to —CF2H are the methods comprising melt-kneading the polymer in a twin-screw extruder while introducing air and water thereinto (cf. e.g. Patent Document 2: Japanese Kokai Publication H12-198813; Patent Document 3: WO 01/018076). The methods disclosed in these documents can give a very high rate of conversion to —CF2H but still have a problem, namely they allow a metal fraction to remain, though in very slight amounts, in the pellets obtained. In addition, the method of Patent Document 3 has a further problem: when the melt flow rate [MFR] of the fluoropolymer is increased, the number of terminal groups increases, so that the stabilization by conversion to —CF2H becomes insufficient unless the alkali addition level is increased.
The metal residue may possibly cause degradation and decomposition of the fluoropolymer at high processing temperatures. This decomposition may possibly lead to discoloration and degradation as well as nozzle blocking. In electric wire covering molding, these cause molecular fraction on the nozzle outlet surface, which negatively affects the process. The so-called cornbreak, namely breaking or cracking of the covering, can also occur. Once cornbreak has occurred, it is inevitable to stop the covering process and then restart the process and, further, wait for a certain period of time until the system arrives again at the equilibrium. This makes it difficult to continue the process for a prolonged period of time, reducing the productivity (cf. e.g. Patent Document 4: Japanese Kohyo Publication 2002-535456).
A method for stabilizing the carboxyl group without using any alkali metal or other metal has been proposed which comprises treating a fluoropolymer as obtained after polymerization with fluorine gas (cf. e.g. Patent Document 5: Japanese Kokoku Publication S46-23245). However, this method requires equipment for fluorination and for removal of fluorine gas and, in addition, fluorine gas itself is highly toxic, producing a safety problem.    Patent Document 1: Japanese Kokoku Publication 537-3127    Patent Document 2: Japanese Kokai Publication H12-198813    Patent Document 3: WO 01/018076    Patent Document 4: Japanese Kohyo Publication 2002-535456    Patent Document 5: Japanese Kokoku Publication S46-23245